1. Field of the Invention
The invention relates to a method for processing spent ion-exchange resins, and in particular, to a method for processing spent ion-exchange resins by reducing the volume of the spent ion-exchange resins through a wet oxidation process and using barium hydroxide to adjust pH of the wet oxidation reaction solution. The method of the invention is applicable mainly to various cleaning treatments in nuclear power plants and in other relative fields.
2. Description of the Prior Art
Current nuclear power plants are operated based on light water reactors that use light water as the coolant of the reactor and as the neutron moderator. If there is corrosive ion present in the reactor water, the corrosion to the reactor will increase and activation products may be produced such that the radioactivity in the reactor system may increase to an extent that the neutron economy might be lowered due to the out of control of neutron capture reactions. In order to overcome these potential problems, ion-exchange resins had been used extensively in the nuclear power plant, including, for example, cleaning treatment of reactor water for supplying de-mineralized reactor water; removing from the reactor water contaminants such as neutron activation products and fission products that are escaped from nuclear fuel elements; lowering the oxygen content in the coolant; controlling contents of the corrosion inhibitor and chemical additives, and the like. Further, ion exchange resins are useful as well in the treatment of wastewater from the nuclear power plant.
For increasing the effective surface area, powdery ion exchange resins other than bead resins are also used in the nuclear power plant. The specific surface area of the powdery ion exchange resin is approximately 100 times than that of the bead ion exchange resin, and it is a kind of very fine powder. They are used usually for coating the filter elements where, in addition to the function of ion exchange, they can remove small solid beads suspended in the water.
After declining of the ion exchange capability of the ion exchange resins or increasing of the radioactivity due to contamination of nuclides in the ion exchange resins, the bead ion exchange resin must be regenerated. After being regenerated and re-used for allowable times, the bead ion exchange resin should be discarded as a radioactive waste. On the contrary, since the powdery ion exchange resin will retrain a great amount of solid impurities therein once in use, it is no more subjected to regeneration and is discarded as waste after being used once. According to the survey conducted by US Electric Power Research Institute for 1982-1985, regarding the fraction of spent resins in wet waste, resin waste from the Boiling Water Rector (BWR) nuclear power plant contained 50% of powdery resin and 25% of bead resin; while resin waste from Pressurized Water Reactor (PWR) nuclear power plant contained 7% of powdery resin and 44% of bead resin. As in Taiwan, resin waste from BWR nuclear power plant contained about 30% of powdery resin and about 11% of bead resin; while resin waste from PWR nuclear power plant contained about 1% of powdery resin and about 23% of bead resin.
The spent ion exchange resin needs to be stabilized for the safety of final disposal. For this purpose, it is usually solidified with agent such as cement, polymers, pitch and the like. The performance of the solidification is characterized differently by the kind of agent used. In general, solidification with high polymers or pitch could result in less volume of the solidified products. However, polymers are expensive. The operation cost with polymers is eventually very expensive. In the other hand, pitch gives lower strength of solidified product and the product is combustible. In March 1997, a fire accident in Japan had been arisen due to solidification operation with pitch, which resulted in a severe nuclear accident. Solidification with cement is a simple and cheap operation. However, since the spent ion exchange resin after solidification still has ion exchange ability such that it can exchange with the calcium ion and the like in the solidified product to the extent of affecting its quality. Further, the solidified resin can absorb or release moisture and hence undergoes swelling or contraction. These phenomena will occur particularly in the bead ion exchange resin that, to the severe extent, might result in the swelling or cracking of the solidified product. All of these will limit greatly the loading rate of the spent ion exchange resin in the cement-solidified product such that a bulky solidified product might be yielded after solidification. In view of the increment of the final disposal cost, the direct solidification of spent ion exchange resin is no more economical.
With the treatment of spent ion exchange resins, two objects are expected mainly as volume reduction and stabilization. Processes for treating spent ion exchange resins may be classified into two types, i.e., the dry process and the wet process. The dry process includes incineration, vitrification and pyrolysis, while the wet process includes acid hydrolysis, oxidative hydrolysis and supercritical water oxidation etc. Of these, incineration process is the earliest developed one and had been implemented commercially in some countries. For incineration, it is done typically in a manner of mixing the spent bead ion exchange resins with other combustible waste in order to control the release concentration of SOx, NOx or other hazardous gas. Release control of radioactive nuclides is also a critical problem to be solved in incineration process. Unless the discharge of volatile nuclides such as, for example, carbon-14, tritium, cesium-137 and the like can be avoided, nuclides must be removed at first from the spent resin to lower its radioactivity before incineration.
In vitrification process, the corrosion of the material is the main problem, since the vitreous molten mass of ion exchange resins could exhibit strong corrosion activity at high temperature. Currently, a variety of melting furnace techniques for vitrification is under development. Among these, the cold wall crucible fusion process developed by SGN Company of France has less corrosion problem and is said could be a promising vitrification technique. The Molten Metal Technology (MMT) of USA had developed a Quantum-Catalytic Extraction Process (Q-CEP) that, in a closed reactor, all the volatile components in ion exchange resins were decomposed into simple gases with molten iron having strong reducing activity, while non-volatile components were left as slag. In this way, effective volume reduction and stabilization could be obtained. The final solidified product in that process is a high active iron block containing sulfur, silicon, sodium and the like, and a special container is required for containing the same. The construction cost of the Q-CEP system is very high and it is not economic to construct a process plant with small capacity system. Further, the treatment left of the considerable amount of gaseous byproducts such as hydrogen sulfide and the like produced by that process could be a difficult problem.
ThermoChem Inc. of USA had developed a steam reforming process, comprising pyrolysis of ion exchange resins under elevated temperature and, meanwhile, generating a combustible gas useful as a fuel through reforming. This process has also a severe problem of materials corrosion, and its future needs to be demonstrated further. A process at its starting step of development is the SuperCritical Water Oxidation (SCWO) process. When decomposition products such as, for example, sulfuric acid, of ion exchange resins are present, the supercritical water of high temperature and pressure employed in that process exhibits similarly a high degree of corrosion that renders the material corrosion a severe problem. That process is still far away from practical use.
The British AEA Industry had developed successfully a wet oxidation process that is comprised of carrying out an oxidative decomposition of bead type spent ion exchange resins by using ferrous sulfate as the catalyst, aqueous hydrogen peroxide as the oxidant and lime hydrate as pH regulator at a temperature of about 100° C. and pH of 3-4 to decompose organic components into CO2 and H2O. It is reported that, for treating 40 kg of ion exchange resins, this process would consumed 160 kg of 50 wt % aqueous hydrogen peroxide, 1 kg of concentrated sulfuric acid, 6 kg of slime hydrate, and less than 0.5 kg of de-foaming agent.
Spent ion exchange resins generated in nuclear power plant includes the cation exchange resin of strong acid type and the anion exchange resin of strong basic type, each possessing different chemical properties. When carrying out a wet oxidation on a cation exchange resin of strong acid type and an anion exchange resin of strong basic type, the over-all reaction can be represented as eq. (1) and (2) respectively:C8H8SO3+20H2O2→8CO2+23H2O+H2SO4  (1)C12H19NO+31H2O2→NH4OH+12CO2+38H2O  (2)As shown in the above two equations, as the ion exchange resin is oxidized, hydrogen carbon constituents are oxidized into CO2 and H2O. According to equation (1) and (2), one mole each of sulfonate group and quaternary amino group contained respectively in cation and anion exchange resins will be oxidized into one mole each of sulfuric acid and ammonium hydroxide, respectively. Sulfuric acid generated from the oxidation of cation exchange resins may, other than increasing the acidity of the solution, impart high corrosive property on the wet oxidation solution due to the presence of both sulfuric acid and hydrogen peroxide. This corrosive property may increase as the progress of the wet oxidation of the cation exchange resin. The wet oxidation of mixed cation and anion exchange resins produces both sulfuric acid and ammonium hydroxide, and therefore, involved also the following reaction between them, i.e.:H2SO4+2NH4OH→(NH4)2SO4+H2O  (3)
The mole ratio of sulfuric acid to ammonium hydroxide in the waste solution of the wet oxidation will vary depending on the mole ratio of the cation exchange resin to the anion exchange resin that have undergone the wet oxidation. When the mole ratio of anion/cation exchange resin is equal to 2, the ammonium hydroxide and sulfuric acid produced will form into ammonium sulfate exactly corresponding to that ratio, and the pH of the solution will increase slightly at the end of the reaction. On the other hand, as the mole ratio >2, there will be excess of ammonium hydroxide; the pH of the waste wet oxidation solution will increase significantly. On the contrary, when the mole ratio <2, there will remain excess sulfuric acid and, accordingly, the pH will lower considerably.
In order to control the pH of the wet oxidation solution, addition of an acid or base might be necessary for adjusting as desired. Typically, lime (calcium hydroxide) or sodium hydroxide is added as the base. Addition of calcium hydroxide will convert sulfuric acid into calcium sulfate and results in the waste solution of wet oxidation contains mainly calcium sulfate. During the solidification of this waste solution, the calcium sulfate will react with 3CaO.Al2O3 present in the cement, forms slowly ettrigite having low density, and renders the solidified product swell gradually to crack eventually, and consequently, a severe quality problem is caused. For alleviating this problem, loading rate of waste in the solidified product must be greatly lowered. This, however, will result in the great increment of the volume of the solidified waste. In addition, calcium sulfate in the waste solution is prone to crystallize and agglomerate to an extent of blocking the transportation pipes, deteriorating transportation and constituting a difficulty in operation.
Also, in the wet oxidation process of spent ion exchange resins, there is an aqueous condensate generated from the cooling of vapors from the wet oxidation reactor in certain quantity as much as that of H2O2 consumed. This condensate may contain a certain amount of nuclides, and also inevitably contain a small amount of organic substances, which, if not treated appropriately, may cause excessive high concentrations of nuclides and total organic carbon (TOC) to an extent that the condensation water could not be discharged or reused.
In view of the foregoing, current wet oxidation process for treating spent ion exchange resins possesses the major disadvantages including: (1) the reaction solution to be treated exhibits very high corrosion property that might cause a severe corrosion attack on the material; (2) the liquid waste generated contains mainly sulfuric acid and its salts that can not be solidified readily; thus the solidified product produced from the liquid waste is bulky that it might counterbalance almost the volume reduction effect achieved by the oxidation of ion exchange resins.
In view of overcoming the above-mentioned disadvantages, after studying extensively the inventor of this application had developed the invention accordingly.